It is known that the production of prestressed reinforced concrete elements with "adhering wires" (particularly in the form of elongated elements, as beams, rafters, cakes) is carried out throughout the world, since several decades, with very similar methods; the characteristics of a conventional plant and of its working can be summed up as follows:
(a) A track or bench is prearranged--which can be a simple floor, usually of considerable length (for instance, from 40 to 200 m)--at both ends of which there are provided anchorage and reaction devices capable of standing horizontal forces.
(b) The loose reinforcements, the inserts, the forms, the separation elements between successive concrete units and whatever else is needed before the casting, are arranged along the whole track in the required amount and positioning.
(c) Prestressing wires (or plaits, or strands) are stretched out along the whole track, in a number and positioning corresponding to the requirements of the concrete units being produced. Said wires are fixed to the anchorage devices and tensioned, at the stress values foreseen according to the predetermined prestress conditions of the concrete units being produced.
(d) Concrete casting is performed.
(e) The concrete is left to cure for the required amount of time.
(f) After opening or removing the forms, the tension on the wires in correspondence of the end anchorage devices is gradually released, thereby prestressing the prefabricated elements.
(g) The wires between each concrete unit are cut, thus finally separating them.
(h) The finished concrete units are removed from the track and stored.
(i) The track and the forms are properly cleaned, prearranging the plant for a new production cycle.
From the above description it is quite evident that the traditional manufacturing method has a marked feature of discontinuity, mainly due to the fact that the step outlined in (e) determines a practically complete stoppage of the plant for a period of at least 8 to 12 hours, that is, the time actually required for the curing of the concrete. The fact that one should succeed, in many cases, in operating so that the curing period should coincide with the overnight stoppage of the plant, is certainly not apt to reduce the discontinuousness of the production.
Nevertheless, the production drawbacks of such a manufacturing technique are not limited to said feature of discontinuity, but are also tied to the presence of a high number (even 400) of concrete units being produced on a single track, and of a large number of expensive self-propelled machines being used for the different operations, but remaining out-of-work for a long time as they each operate in turn for a period usually not exceeding 45 to 75 minutes throughout the daily production cycle. On the other hand, said machines must have a high capacity in order to perform their work in a short length of time. For example, the machine for preparing concrete should have a capacity of at least 30 m.sup.3 /h, though working for about 60 to 70 minutes a day: concrete casting in the forms must in fact be rapidly carried out, since the curing period (which has to pass before loosening the anchorage devices and which, as far as production is concerned, is a dead period) starts to run from the casting of the last form.
It is also evident that, by allowing the curing to take place overnight--thus organizing the production cycle so that casting is performed at the end of the normal day shift--it could happen that a failure or other inconvenience preventing to complete the casting could determine an overall postponement of the curing period, thereby causing heavy production losses, upsetting of the working times and difficulties in resetting the normal working cycle.
The object of the present invention is to realize a plant for the production of elongated elements of prestressed reinforced concrete, particularly railway sleepers of prestressed concrete, apt to eliminate the aforementioned drawbacks, to drastically reduce plant and production costs, to make a more rational use of labour and of the operating machines, and to achieve a sufficient continuity of the manufacturing process.
A plant apt to satisfy some of these requirements is described for example in the GB-A-572 390, wherein the forms are mounted sliding along parallel tracks and the tensioning reinforcements, particularly the tensioning wires, are fixed to the ends of the plant, the forms being thus slidable along the same. A serious drawback of this plant is nevertheless actually determined by the fact that the forms slide in respect of the tensioning wires, whereby the wires practically move through the concrete cast in the forms which is already starting to set; this first of all causes problems of final bond (adherence) between the reinforcements and the concrete mass. Furthermore, also the operation of the plant is quite complicated, since the forms of one track and the respective reinforcements stretched out through the plant remain in position for the whole concrete curing period, thereby interferring with the operations being carried out on the forms of another track.